Containers for retail packaging of roast and ground coffee are often equipped with a one-way valve to permit egress of carbon dioxide (CO2) while substantially preventing ingress of air. Some such containers comprise a self-supporting, or rigid, plastic body, sealed with a peelable film cover, and a snap-on, snap-off plastic overcap. When such containers are transported from packaging plants to retail stores, they are sometimes transported over mountains at high enough altitudes (e.g., altitudes around 7000 feet) that significant internal pressure drops are experienced. In some cases, a drop of about 5 psi may occur during transport while the container is at high altitude, and upon returning to a lower altitude, the internal pressure will remain at about 5 psi below the external pressure, which may be, e.g., about 5 psi below atmospheric pressure at sea level. This can result in significant stress on the container seal(s).
For packages that comprise a rigid container with a removable peelable film cover, or film membrane, the strength of the seal that holds the cover to the container can be an issue. A wide sealing surface or rim can be advantageous in terms of providing more sealing area, and an outwardly extending structure at or near the top of the container may be used to facilitate snap-on engagement of an overcap with the container finish. However, an internal vacuum and the attendant downward force on the peelable film cover may result in radially inward stress on the mouth of the container causing a localized reduction of diameter and interfering with the function of retaining the overcap in snap-on, snap-off engagement, particularly in relatively thin-walled, lightweight containers, such that the vacuum forces substantially affect the lid fit.
When intermodal shipment of containers over mountains produces an internal vacuum within the containers, the vacuum can pull the sealing membrane down tightly around the opening of the container. Where the membrane is sealed to the container the downward force of the membrane can also pull the plastic walls of the container inward, such that the sealing surface is pulled downward and inward. Since the overcap engages the container high upon the neck finish, in the area of the sealing surface, this reduces the diameter of the outwardly extending structure (e.g., snap bead segments) that engages the overcap, thus resulting in a loose fit of the overcap.
Traditionally, the sealing surface of wide mouth extrusion blow molded bottles have a flange that projects inward and/or outward at the top of the finish to provide a sealing surface. The finish thus has a cross-section resembling an inverted “L” at the top. The wide-mouth sealing surface can have a flange width or sealing width that may range from 0.125 to 0.16 in. The flange overhanging the rim of the container is typically fairly flexible, and can flex or bend under pressure from a sealing head. Flexing can be desirable as it can help to allow the seal head to conform to the sealing surface and effect the seal. However, it can also be undesirable because it can result in the sealing surface bending too much, resulting in a poor contact area when the seal head comes down to effect the seal between the sealing membrane and the flange. This can result in a poorly sealed or unsealed membrane, which can result in the coffee being open to the atmosphere and cause premature staling of the coffee.
An inwardly extending flange at the sealing surface can also interfere with the pouring of coffee from the interior of the container.
Another problem with some prior containers is that they sometimes have nicks, uneven surfaces, dips, visible scars or other abrupt changes in the height or smoothness of the sealing surface. These inconsistencies in the surface can also interfere with creating a hermetic seal of the sealing membrane to the sealing surface. A secondary finishing treatment, e.g., burnishing or machining, may be used to smooth the sealing surface; however, this results in an additional cost to the process as well as reducing the line efficiency in production. Sealing materials, e.g., Surlyn® from DuPont, can also be used as a “caulk” to attempt to fill in the interruptions, gaps and/or uneven surfaces in the sealing surface; however, these attempts at filling in the interruptions, etc. often are not successful.
Alternatively, a flat sealing surface can be obtained by the injection blow molding process; however, the injection blow molding process is not well suited to making multilayer bottles, e.g., from multilayer materials such as high density polyethylene (HDPE)/ethylene vinyl alcohol (EVOH)/HDPE, and is not well suited to making bottles with handles.
Additionally, the wide sealing surface due to the inward and outward flanges can require a higher removal force to remove the sealing membrane. This can be especially undesirable for consumers who have difficulties grasping and pulling on the membrane tab to open. Bottles or containers having a straight wall finish would provide a smaller sealing surface which can result in application of a lower removal force to remove the sealing membrane.